Leukemia may result from growth factor-independent growth of progenitor cells, loss of response to differentiative signals, and/or loss of sensitivity to negative growth regulators. In the last few years, it has become clear that activation of oncogenes are fundamental to the development of both cancer and leukemia. Using the v-src oncogene, we have been examining how oncogenes alter the proliferation and differentiation of hematopoietic cells. In several different systems, we have observed that oncogenes can induce the production of hematopoietic growth factors. This can result in autocrine or paracrine growth of hematopoietic progenitor cells, which may contribute to the development of leukemia. In this proposal the mechanism by which oncogenes induce production of hematopoietic growth factors will be examined in two different model systems; first, in bone marrow stromal cells where a paracrine growth cycle increases proliferation of hematopoietic progenitor cells in long term bone marrow cultures, and second, in factor-dependent myeloid cell lines where oncogenes establish an autocrine growth cycle. Alternations at the level of transcription will be examined by determining the levels of GM-CSF RNA and by nuclear runoff studies. Potential post-transcriptional modification to be examined include changes in the stability of the GM-CSF mRNA, changes in the rate at which the GM-CSF protein is processed and secreted, and alterations in the association of GM-CSF with the stromal cell extracellular matrix. Once the mechanism(s) responsible for increasing production of GM-CSF is identified, we will attempt to identify the cellular proteins that are modified by src, perhaps by phosphorylation, which regulate this change. Understanding the molecular basis of autocrine and paracrine growth cycles will provide fundamental knowledge about normal cellular processes and how alterations in these processes can contribute to leukemogenesis and/or carcinogenesis. It is anticipated that understanding the molecular basis for induction of autocrine and paracrine growth cycles may lead to new modalities of treatment for both cancer and leukemia.